manual: add town square and gemstone bonuses

MANUAL.html

@@ -169,6 +169,18 @@
     <tr><td> Road                 </td><td> 8-36     </td></tr> 
     <tr><td> Research             </td><td> 24-60    </td></tr> 
   </table>
+  <br>
+  Some bonuses apply:
+  <ul>
+    <li>
+      Buildings of type cultural, affluent residential, luxury residential and commercial get
+      an extra bonus of up to 0.15% for each square touching the town square, up to 15%.
+    </li>
+    <li>
+      Buildings of type cultural, affluent residential and luxury residential get extra bonuses
+      if their owner owns gemstones (the rarer the gemstones, the better the bonuses).
+    </li>
+  </ul>
 </p>
 
 <h3> Shares </h3>